Field-effect transistors (FETs) include a gate electrode, a source electrode, and a drain electrode, and are electronic active devices that control electric current between the source electrode and the drain electrode by controlling the flow of electric current into a channel layer through voltage application to the gate electrode. FETs that use as the channel layer a thin film deposited on an insulated substrate such as a ceramic, glass, or plastic substrate, in particular, are called thin film transistors (TFTs).
The above-mentioned TFTs are formed by using a thin film technology, and hence the TFTs have an advantage of being easily formed on a substrate having a relatively large area, and therefore are widely used as a driving device for a flat panel display such as a liquid crystal display. Specifically, an active liquid crystal display (ALCD) turns on/off each image pixel by using TFTs formed on a glass substrate. Further, for a future high performance organic LED display (OLED), it is effective to control current of each pixel by TFTs. In addition, a liquid crystal display having a higher performance is realized in which peripheral circuits having a function of driving and controlling an entire image is formed on a substrate in the vicinity of an image area by using TFTs.
The most popular TFTs at present are ones that employ a polycrystalline silicon film or an amorphous silicon film as a channel layer material. Amorphous silicon TFTs for pixel driving and high performance polycrystalline silicon TFTs for overall image driving/controlling have been put into practical use.
A drawback of TFTs developed in the past, including amorphous silicon TFTs and polysilicon TFTs, is that a high-temperature process is required in manufacturing those devices, which makes it difficult to form the TFTs on a plastic plate, a film, or other similar substrates.
Meanwhile, the development of flexible displays in which a TFT formed on a resin substrate such as a polymer plate or a film serves as a drive circuit of an LCD or of an OLED has become active in recent years. This draws attention to organic semiconductor films, which can be deposited at low temperatures and have electrical conductivity, as a material that can be deposited on a plastic film or the like.
Pentacene is an example of organic semiconductor films, and its research and development is being advanced. It has been reported that the carrier mobility of pentacene is about 0.5 cm2/Vs, which is equivalent to the carrier mobilities of amorphous Si TFTs.
However, pentacene and other organic semiconductors have problems of being low in thermal stability (<150° C.), and have not succeeded in producing a device usable in practical uses.
Another material that has recently been drawing attention as being applicable to the channel layer of a TFT is an oxide material. For example, TFTs using as the channel layer a transparent conductive oxide polycrystalline thin film having ZnO as a major component are being developed actively. This thin film can be deposited at relatively low temperatures and formed on a plastic plate, a film, or other similar resin substrates. However, in general, a compound having ZnO as a major component cannot form a stable amorphous phase at room temperature and forms a polycrystalline phase instead, which causes electron scattering in the polycrystalline grain boundaries and makes it difficult to increase the electron mobility. In addition, electrical properties of such a polycrystalline compound is greatly influenced by shape and interconnection of polycrystalline grains, which could depend on the fabrication process of a film deposition condition etc., and hence resultant TFT devices have fluctuating characteristics in some cases.
With regard to this problem, a thin film transistor that uses an In—Ga—Zn—O-based amorphous oxide has been reported in K. Nomura et al., Nature vol. 432, pp. 488-492 (2004-11). This transistor can be formed on a plastic or glass substrate at room temperature. The transistor also accomplishes the characteristics of a normally-off transistor at a field-effect mobility of about 6 to 9. Another advantageous characteristic is that the transistor is transparent with respect to visible light. Specifically, the above-mentioned document describes a technique of using an amorphous oxide that has a composition ratio of In:Ga:Zn=1.1:1.1:0.9 (atomic ratio) for the channel layer of a TFT.
While an amorphous oxide using three metal elements In, Ga, and Zn is employed in the above-mentioned document, it is better in terms of ease of composition control and material adjustment that a smaller number of metal elements are used.
On the other hand, oxides that use one type of metal element, such as ZnO and In2O3, generally form polycrystalline thin films when deposited by sputtering or a similar method, and accordingly are likely to cause fluctuations in characteristics of a TFT device as described above.
Results of study on an In—Zn—O-based amorphous oxide as an example of using two types of metal elements have been known from the report in Applied Physics Letters 89, 062103 (2006). However, it has been known that the resistivity of an In—Zn—O-based amorphous oxide could be varied when the oxide is stored in atmospheric air, and thus improving the environmental is desired.
In addition, results of study on an In—Zn—O-based amorphous oxide have been reported in Solid-State Electronics, 50 (2006), pp. 500-503. In this report, heat treatment of 500° C. is performed, and hence it is desirable to manufacture a device at lower temperatures. This is because, if the device can be manufactured at low temperatures, an inexpensive glass substrate or resin substrate can be used.
Therefore, an amorphous oxide which is comprised of a smaller number of metal elements and has excellent stability is desired in the field of the thin film transistors.